1. Field of the Invention
The present invention relates to an electronic endoscope system for capturing an image of a portion to be observed in a subject using an electronic endoscope.
2. Description Related to the Prior Art
Diagnoses and treatments using an electronic endoscope are common in the medical field. In an electronic endoscope system for capturing an image of a portion to be observed in a subject using an electronic endoscope, normal light observation using white light (hereinafter referred to as the normal light) as illumination light is performed. However, it is often difficult to observe details of tissue under the normal light observation. To observe the details, special light observation has been performed recently. Under the special light observation, an image is captured while narrowband light (hereinafter referred to as the special light) in a specific wavelength range is applied to the portion to be observed. Thereby, it becomes easy to observe details of specific tissue in the image captured. Under the specific light observation, an image shows clear contrast between tissue which absorbs the special light and remaining tissue. Thus, the tissue which absorbs the special light is enhanced in the image.
On the other hand, a technique to improve contrast of specific tissue by applying image processing to an image has been known. Thereby, the specific tissue is enhanced in the image. For example, by applying frequency enhancement processing for enhancing an image of a predetermined frequency, contrast of a blood vessel (hereinafter referred to as the surface blood vessel) located in a mucosal surface or a blood vessel (hereinafter referred to as the subsurface blood vessel) located below the surface blood vessel improves (see Japanese Patent Laid-Open Publication No. 2000-148987).
Under the special light observation, the tissue which absorbs the special light is enhanced. However, remaining tissue observable under the normal light observation is also shown in the image in the same manner as the normal light observation. Accordingly, the tissue which absorbs the special light is superposed on the remaining tissue observable under the normal light observation in the image displayed.
For example, under the normal light observation, a surface blood vessel is superposed on a subsurface blood vessel in an image. When the special light observation is performed, the image of the surface blood vessel, which absorbs the special light, is enhanced in a state that the surface blood vessel is superposed on the subsurface blood vessel. The subsurface blood vessel often hinders the close observation of the surface blood vessel.
Conversely, when the special light observation is performed using special light absorbed by the subsurface blood vessel to enhance the subsurface blood vessel, the subsurface blood vessel is enhanced while being superposed by the surface blood vessel. The surface blood vessel often hinders the close observation of the subsurface blood vessel.
On the other hand, frequency enhancement processing improves contrast of an image of a predetermined frequency. The frequency enhancement processing is applied to an image captured under the special light observation. However, tissue under observation may be enhanced incorrectly depending on a subject distance.
More specifically, when the subject distance is long, a subsurface blood vessel is narrow in an image captured. Accordingly, the frequency enhancement processing, aiming at the surface blood vessel, enhances the subsurface blood vessel instead. Conversely, when the subject distance is short, the surface blood vessel is wide in the image. Accordingly, frequency enhancement processing, aiming at the subsurface blood vessel, enhances the surface blood vessel instead.
Under the special light observation, the surface blood vessel is superposed on the subsurface blood vessel in an image. It is difficult to observe a target blood vessel when a non-target blood vessel is enhanced with the frequency enhancement processing.
To solve the problem, it is desired to improve visibility of the target blood vessel without being hindered by non-target blood vessel(s).